Microwave Spectra, Barriers to Internal Rotation of the Methyl Group, and Molecular Electrical Dipole Moments in <i>trans</i>- and <i>cis</i>-1,3-Pentadiene

Hsu, Shu-Kun; Flygare, W. H.

doi:10.1063/1.1673098

Cited by 35 publications

(4 citation statements)

References 12 publications

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“…For 1,3-pentadiene, geometry optimization reveals three stable conformers, the s-cis,trans, the s-trans,trans, and the s-trans,cis isomers. In agreement with microwave spectroscopic studies, the two s-trans species are predicted to be the most stable ones, which therefore were considered for frequency calculations. Among them, the s-trans , cis -1,3-pentadiene is calculated to be higher in energy by 6.95 kJ/mol than the s-trans , trans- 1,3 pentadiene.…”

Section: Resultsmentioning

confidence: 64%

Calculation of Vibrational Spectra of Linear Tetrapyrroles. 4. Methine Bridge C−H Out-of-Plane Modes

2006

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Quantum chemical force fields obtained by density functional theory (DFT) calculations systematically overestimate the frequencies of normal modes including ethylenic C-H out-of-plane (HOOP) coordinates. Compensation of this deviation requires a specific scaling factor for this type of coordinate that is distinctly lower than those applicable to out-of-plane coordinates in general. Such a specific scaling factor (0.900) has been optimized for the DFT(B3LYP) level of theory on the basis of vibrational analyses of training molecules including the HOOP coordinate. Thus, the root-mean-square deviation for the calculated frequencies of these modes is reduced from 16 to 8 cm(-1). Although Raman intensities are yet not reproduced in a satisfactory manner, implementation of the HOOP scaling factor into the set of global scaling factors determined previously (Magdo et al. J. Phys. Chem. A 1999, 103, 289-303) allows for a substantially improved reproduction of the experimental (resonance) Raman spectra of test molecules including linear methine-bridged tetrapyrroles. A very good agreement between calculated and experimental spectra is noted for the phycocyanobilin dimethylester dimer as well as for the protein-bound phycocyanobilin in the antenna pigment alpha-CPC. However, for the phycocyanobilin chromophore in the P(r) state of the plant photoreceptor phytochrome phyA, considerable deviations remain in the spectral range between 800 and 500 cm(-1), which are attributed to the effect of specific protein-chromophore interactions. The influence of the protein environment is not considered in the present calculations that refer to the molecule in vacuo.

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Section: Resultsmentioning

confidence: 64%

Calculation of Vibrational Spectra of Linear Tetrapyrroles. 4. Methine Bridge C−H Out-of-Plane Modes

2006

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“…All three can in turn exist in different conformations, s - cis , s - trans , or gauche . Evidence from microwave spectroscopy, , electron diffraction, infrared and Raman spectroscopy, and thermodynamic studies , suggests that the most stable conformation of all three compounds is the s - trans . No indication of the existence of a higher energy conformer of trans - and cis -1,3-pentadiene was found by variable-temperature infrared and Raman spectroscopy and thermodynamic studies, while the structure of the less stable conformer of isoprene was shown to be planar s - cis by the same method 41 and also by Raman and ultraviolet absorption spectroscopy.…”

Section: Discussionmentioning

confidence: 99%

Radical Cations of Monomethyl-Substituted 1,3-Butadienes: Resonance Raman Spectra and Molecular Orbital Calculations

Keszthelyi

Wilbrandt

1996

J. Phys. Chem.

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The resonance Raman spectra of the trans-and cis-1,3-pentadiene and the 2-methyl-1,3-butadiene (isoprene) radical cations generated by γ-irradiation in a Freon glass are reported. The spectra are excited in resonance with the first absorption band. The spectra, which are interpreted with the help of density functional theory calculations, indicate the presence of both the s-trans and the s-cis rotamers of all three radical cations.

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“…The CH bonds in the methyl group are the longest CH bond lengths in trans -1,3-pentadiene, hence these CH transitions will occur at the lowest frequency. The methyl group in trans -1,3-pentadiene has a relatively high barrier to internal methyl rotation (1.80 kcal mol -1 equivalent to 629 cm -1 ), which results in the methyl group being effectively locked into position. As a result, we expect to observe two peaks for the methyl group, one for the CH op bonds and one for the CH ip bond, as is often seen in the CH-stretching overtone spectra of methyl groups .…”

Section: Resultsmentioning

confidence: 99%

“…The methyl group in cis -1,3-pentadiene has a significantly lower barrier to internal rotation than trans -1,3-pentadiene. Microwave studies have found the barrier to methyl rotation to be 0.74 kcal mol -1 (258 cm -1 ) for cis -1,3-pentadiene . Rong and Kjaergaard have found that at barrier heights above 450 cm -1 the methyl group is effectively locked in position and the coupling between rotation and CH-stretching is effectively decoupled .…”

Section: Resultsmentioning

confidence: 99%

CH-Stretching Overtone Spectra of cis- and trans-1,3-Pentadiene

et al. 2006

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The room-temperature vapor-phase overtone spectra of cis- and trans-1,3-pentadiene (piperylene) have been recorded in the 5000-17500 cm(-1) region with the use of conventional and intracavity laser photoacoustic spectroscopy. The presence of five nonequivalent olefinic CH bonds and one methyl group in each molecule complicates assignment of the spectra. We have used a harmonically coupled anharmonic oscillator local mode model with one oscillator for each of the nonequivalent CH bonds to calculate the CH-stretching overtone spectra and thus facilitate assignment of the spectra. Our calculated spectra are in good agreement with the observed spectra. The observed high overtone spectra are distinctively different despite the similarity of the two molecules and the numerous and broad transitions.

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Microwave Spectra, Barriers to Internal Rotation of the Methyl Group, and Molecular Electrical Dipole Moments in trans- and cis-1,3-Pentadiene

Abstract: Articles you may be interested inMicrowave spectrum, molecular structure, barrier to internal rotation, and dipole moment of methyl monofluorogermane

Cited by 35 publications

References 12 publications

Calculation of Vibrational Spectra of Linear Tetrapyrroles. 4. Methine Bridge C−H Out-of-Plane Modes

Calculation of Vibrational Spectra of Linear Tetrapyrroles. 4. Methine Bridge C−H Out-of-Plane Modes

Radical Cations of Monomethyl-Substituted 1,3-Butadienes: Resonance Raman Spectra and Molecular Orbital Calculations

CH-Stretching Overtone Spectra of cis- and trans-1,3-Pentadiene

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